Took me a while to get to the bottom of this. Maybe I should have looked at /opt/lampp/logs/error_log earlier and saved myself the ache.
The error I found when I did look was "xdebug.so: wrong ELF class: ELFCLASS64"
A quick lookup on the internet and it turned out that as I'm running on 64 bit Linux I'd compiled the 64 bit version of xdebug.so. What I needed as Xampp runs 32 bit was the precompiled 32 bit version.
I downloaded it from here http://aspn.activestate.com/ASPN/Downloads/Komodo/RemoteDebugging and was running in seconds.
Many thanks to Jonatas Esteves from http://www.sanisoft.com/blog/2007/06/23/how-to-install-xdebug-php-extension-for-xampp-on-linux/ for his excellent post.